it 做形式主语

1.        At a time when it was unusual to do it, Dorothy Sterling wrote about such major figures of Black history as Harriet Tubman and W. E. B. Du Bois.

(A) it was unusual to do it

(B) it was unusual to do so

(C) doing that was unusual

(D) that was an unusual thing to be doing

(E) it was not usual to do                                 

答案是（B）没有问题，但是，it在这里的作用是形式主语吗？